Polycro

Pros

• And durability really isn't much of a concern when a single polycro groundsheet can last like 500 miles
• I pair mine with a polycro floor, which I recommend for general livability
• Polycro is great for cowboying. It's 100% waterproof unlike Tyvek. Keep your gear, your quilt, your pad, your feet/socks relatively clean and dry. Roll up the edges to protect against poor drainage.
• Just use polycro if you want a very light ground sheet.
• I use polycro for short trips, because it *is* lighter, but the number one question I get is "doesn't that tear?" and the answer is "yes."

Cons

• I always manage to tear up the polycro ones within a few weeks
• I trust it as a ground cloth, where a tear isn't that critical. ... I have read that some use it as a tarp, but I would never trust it for that. Sure, it's waterproof, but it wouldn't take much wind to tear it at the tie off points.
• It resists punctures pretty good, but it rips easily.
• My Tyvek is already cut to the tent dimensions. Tried polycro, hated it.